It is well known that surface defects appear during manufacturing of semiconducting substrates such as SOI (Silicon On Insulator) substrates intended specifically for micro-electronic, optoelectronic application, etc.
Said substrates are usually obtained by Smart-Cut™ and by a succession of heat treatments.
During these heat treatments, the substrate is subjected to temperature gradients and a plastic deformation that causes the appearance of surface defects called Slip lines on the active layer that reduce the substrate quality.
SOI substrates are held in place by supports during heat treatments, for example annular supports or supports with three or four bearing points, etc. During high temperature heat treatment steps, for example at a temperature exceeding 1000° C., the retaining support exerts high forces on the substrate and particularly shear stresses. These forces can deform the substrate and thus cause the formation of fracture planes in which the crystalline structure is offset by sliding after fracture, which can be observed in the form of corresponding slip lines. The shift amplitude may be of the order of one nanometer and thus generate steps of the same dimension on the surface. The fracture can cross through a part of the thickness of the substrate, and can sometimes cross through the substrate from its back face to its front face.
For example, high quality SOI type substrates require heat treatments at a temperature of 1100° C. or even more than 1200° C., causing the formation of slip lines, particularly close to contact zones between the substrate and the support holding the substrate in position during heat treatments.
Thus, an annular support typically shows up slip lines around the periphery of the substrate, while a support with three bearing points will make them appear in substrate bearing zones arranged facing the bearing points. The slip lines will deteriorate the quality of the monocrystalline silicon on insulator layer and consequently electronic components that are made later.
Such defects are not observed only on SOI type hetero-structures, they also occur on other types of substrates, for example homogenous monocrystalline silicon substrates.
In order to maintain a good quality of marketed substrates, the usual practice is to examine the surface of these substrates to determine any slip lines that occur during manufacturing.
Fast and reliable detection of all slip lines using a single detection technique is found to be very difficult.
If several techniques commonly used in industry are used and the results are compared, it can be seen that there is no technique capable of identifying all slip lines and in particular slip lines that occur at the periphery of the substrate. Techniques based on detection of beams of reflected and/or scattered light at the surface of the substrate cannot be used around the edge of the substrate where the wafer has a rounded edge such that light cannot be collected and therefore the measurement cannot be made. Therefore the substrate is not analysed over several millimetres around its periphery.
Therefore, an attempt is made to find a detection tool capable of identifying all slip lines and other types of defects that can be detected by optical methods such as holes present in the surface, etc., in a single measurement.
There are powerful and complex laboratory devices that admittedly are capable of detecting practically all defects that can be detected optically, such as slip lines or holes present at the surface, etc.
However, these devices are very expensive and substrates can only be analyzed at a rate of about 7 substrates per hour.
A rate of 50 or even 100 substrates per hour would be necessary to be compatible with industrial production.
Most methods for the detection of slip lines on such substrates consist of projecting polarized coherent light emitted by a laser source onto the substrate, and using a photo-detector to detect light reflected by the substrate. When the incident beam is reflected on the surface of the substrate, light is scattered by the defects. The photo detector detects and quantifies this light scattered in this way.
Such methods are described for example in Japanese patents JP4042945 and JP60122358.
These methods have the disadvantage that they require a particularly long development time and their efficiency is not good. These methods are incapable of detecting low amplitude slip lines. In particular, these methods are incapable of detecting slip lines present on the edges of the substrate (the rounded edges make it impossible to satisfactorily collect reflected light), although slip lines are particularly frequent at edges.
Japanese patent JP3150859 also describes a method for detection of slip lines in a semiconducting substrate in which said substrate is placed under a differential interference microscope provided with a television camera connected to a converter. Said converter transforms the video signal from the camera into the form of an electrical current that is compared with a limiting defect detection current. The substrate is displaced along at least two directions orthogonal to the microscope centre line, so as to check the entire substrate surface.
This method has the disadvantage firstly that it can only be used to detect large slip lines, and secondly, that the processing time for a substrate according to this method is long and results in a limited supply rate of analysed substrates.
Japanese patent application JP 2001/124538 discloses a method and a device for detecting defect which occurs in the surface of a semiconductor wafer, such as slip lines. The method includes a step of projecting a pattern composed of an alternation of light fringes and dark bands on the substrate, so as to generate fringes reflected by the surface of the substrate. Subsequently, images of the pattern reflected by the substrate are captured by a CCD sensor and defects are visually detected on a display screen.
This method presents the disadvantage to be inefficient to detect small defects and to be prone to human error.
Prior art US 2001/0033386 discloses an optical system for detecting surface defects on an object including a step of projecting a pattern composed of an alternation of light fringes and dark bands on the substrate, so as to generate fringes reflected by the surface of the object, a step of relative displacement of the pattern and the object, a step of acquisition of a sequence of three images of the pattern reflected by the object, and a step of determination of the height as a function of the imaging according to U.S. Pat. No. 6,750,899. The height is determined for each pixel of the images in function of the variation of the relative phase of the fringes.
This method presents the disadvantage to be inefficient to detect small defects too.
Moreover, not all these methods can efficiently detect so-called non-transferred zones that can occur during a manufacturing process comprising a step to transfer a layer and then a step to detach the layer using SmartCut™ process. These ZNT zones are well known to those skilled in the art and correspond to regions in which molecular bonding is not as effective as the detachment mechanism, such that the layer is not transferred in these regions. ZNT zones at the edge of the substrate are not always detected, particularly in a zone in which measurements cannot be made due to the rounded edge of the substrate. It is also found that above a given size, these techniques are not very effective at detecting ZNT zones.